HIV-1 infections have caused a worldwide pandemic that has claimed the lives of over 20 million people and infected over 40 million more. In addition to its importance in public health, HIV-1 and other retroviruses have also become important tools for delivery of foreign genes into target cells for gene therapy and other applications. During the assembly process HIV-1 must acquire a viral surface glycoprotein to target and fuse with host cells. Although HIV-1 usually acquires it own surface glycoprotein, it can also assemble into infectious virions with certain foreign viral glycoproteins to form what is called a pseudotyped virus. However, not all foreign glycoproteins are compatible with HIV-1. The glycoprotein from gammaretrovirus gibbon ape leukemia virus (GaLV Env) is an example of a glycoprotein that is not compatible with HIV-1. Co-expression of HIV-1 with GaLV Env yields essentially no infectious pseudotyped virus particles. The element in GaLV responsible for this incompatibility has been mapped to the C-terminal cytoplasmic tail domain (CTD) of the glycoprotein. We recently discovered that the incompatibility between HIV-1 and GaLV Env is modulated by the HIV-1 accessory protein Vpu. In the absence of Vpu, glycoproteins with a GaLV CTD are incorporated into HIV-1 particles, and the particles are infectious. However, in the presence of Vpu, these glycoproteins are not incorporated into HIV-1 particles, and the particle infectivity is reduced by 50 to 100 fold. In some ways this phenomenon is similar to the known Vpu activity of targeting the host cell protein CD4 for degradation at the endoplasmic reticulum. However, the mechanism of GaLV Env restriction is distinct from current models for how CD4 is restricted. Most nobly, Vpu only restricts certain retroviruses from acquiring glycoproteins with a GaLV CTD, and the restriction occurs even in presence of significant surface glycoprotein expression. Although Vpu is known to modulate certain host cell proteins to enhance viral infectivity, it is surprising that Vpu specifically modulates the surface glycoprotein from a foreign virus that displays no obvious similarity to Vpu's known targets. We therefore hypothesize that GaLV Env restriction by Vpu is an incidental effect resulting from GaLV Env mimicking a cellular protein and 'tricking'Vpu into targeting it. By characterizing this downmodulation, we can (1) elucidate the actions of Vpu in infected cells and (2) identify the precise motif in MLV/GaLV Env targeted by Vpu, yielding the necessary information to identify natural Vpu targets in silico. PUBLIC HEALTH RELEVANCE: HIV-1 infections have caused a worldwide pandemic that has claimed the lives of over 20 million people and infected over 40 million more. It has long been recognized that viruses, including HIV, have distinct mechanisms for modulating the host cell to facilitate virus assembly, but these mechanisms remain poorly understood. An understanding of what and how HIV-1 accessory proteins modulate proteins in the infected cell could lead to new targets for antiviral therapies.